new lint: unneccessary usage of String::from() involving string constants

let length = String::from("some constant").len();

can be replaced by

let length = "some constant".len();

The second one will actually emit less code:
String::from().len()

  lea rsi, [rip + str.0]
  lea rdi, [rbp - 64]
  mov edx, 13
  call <alloc::string::String as core::convert::From<&'a str>>::from@PLT
  mov rax, qword ptr [rbp - 48]
  mov qword ptr [rbp - 16], rax
  mov rsi, qword ptr [rbp - 56]

.len()

  sub rsp, 96
  mov qword ptr [rbp - 8], 13

according to godbolt rust/ 1.23.0 with opt-level=3

Start by reproducing the reported examples with Rust 1.23.0 and compare the generated code, using the issue's godbolt reference as the baseline. Done means a Clippy lint identifies String::from() around string constants and suggests using the string literal directly, with coverage for the shown cases.